Lanthanum modification of crystalline phases and residual glass in augite glass ceramics produced with industrial solid wastes

Journal of Non-Crystalline Solids 524 (2019) 119638

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Lanthanum modification of crystalline phases and residual glass in augite glass ceramics produced with industrial solid wastes

T

⁎

Hua Chena, Baowei Lib, , Ming Zhaob, Xuefeng Zhanga, Yongsheng Dua, Yu Shib, John S. McCloyc a

College of Science, Inner Mongolia University of Science & Technology, Baotou, China Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science & Technology, Baotou, China c School of Mechanical & Materials Engineering, Washington State University, Pullman, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords:

The effects of La2O3 on microstructure and properties of the CaO-MgO-SiO2-Al2O3 glass ceramics, made from Bayan Obo West Mine tailing and fly ash, were investigated. Results showed that La2O3 additions up to 2 wt% promoted augite crystallization. Above this concentration, La2O3 suppressed augite crystallization by rather forming lanthanum oxyapatite that acted as a barrier for the growth of augite. Also Cr ions, the key element for the augite nuclei, were enriched in lanthanum oxyapatite. The La3+ cations were found to partially substitute for Ca2+ at the M2 structural sites in clinopyroxene (augite). Nano crystalline clusters, containing La3+, Na+, Ca2+, Mg2+ and Fe3+ in augite-like proportions, were found uniformly dispersed in the residual glassy phase. The glass ceramic with 2 wt% La2O3 shows optimum mechanical properties and chemical durability. The bending strength and Vickers hardness are 238 MPa and 7.57 GPa, acid and alkali-resistance are 97.5% and 98.9%, respectively.

Lanthanum

oxide

Augite Glass

ceramics glass Crystallization Residual

1. Introduction Glass ceramics with augite crystals, a pyroxene mineral incorporating a wide range of cations, normally have good mechanical properties and high chemical durability [1,2]. Therefore, augite is often the preferred crystalline phase for glass ceramics created using industrial solid waste as raw materials, such as metal tailings [3], fly ash [4], blast furnace slag [5], etc. [6]. In addition, these industrial solid wastes mainly contain SiO2, CaO, MgO, Al2O3 and other metal oxides [7], which are the main components of the chemically flexible crystal phase augite [8,9]. There are many variations in composition of augite in these glass ceramics, because ion substitution is very common in the augite phase [10,11]. The ideal structural formula of pyroxene, the general class for minerals like augite, is M2M1Z2O6 [12]. M1 normally hosts a smaller cation, and is coordinated as a regular octahedron, therefore transition metals such as Mg2+ and Fe2+ prioritize this site. The M2 site is either a distorted octahedron (for orthopyroxenes) or a highly distorted site with coordination number (CN) 6 (octahedral) to 8 (for clinopyroxenes like augite); large alkali and alkaline earth cations are normally preferred in the M2 site. Z sites are tetrahedrally coordinated cations which are normally glass-formers or intermediates, such as Si4+ and Al3+ [13]. For augite glass ceramics, high crystallinity always means good

⁎

mechanical and chemical properties. Adding nucleating agents is one way to ensure high crystallinity. Cr2O3 is a typical nucleating agent for CaO-MgO-SiO2-Al2O3 (CMAS) glass-ceramics [14]. Chromia reacts with MgO, Al2O3, Fe2O3 or FeO to form spinel (MgCr2O4) [15] or spinel-like phases (Mg,Fe)(Al,Fe,Cr)2O4 [16], which nucleate augite or other crystalline pyroxene minerals. Lanthanum oxide, a well-known network modifier in glass [17], can also be used as nucleating agent in glass ceramics. Salwa [18] reports that La2O3 nucleates magnetite for magnetic Fe2O3-CaO-ZnO-SiO2B2O3-P2O5 glass ceramics. Besides its nucleating effect, some reports show that La2O3 affects the microstructure of a glass in several different ways [19]. For example, La3+ can substitute for Sr2+ in the (Ba,Sr)TiO3 lattice in barium strontium titanate glass ceramics, though 1.0 mol% La2O3 is beyond the limit of the lanthanum substitution into the (Ba,Sr)TiO3 lattice [20]. La3+ also tends to form La-O-La crystal clusters owing to its high field strength and high coordination number [21]. In borosilicate glasses, La3+ can incorporate into oxyapatite with alkali and alkaline earth ions [22]. In Na2O–BaO–Nb2O5–SiO2 glass ceramics, La3+ substitutes for Ba2+ in the tungsten bronze phase and Na+ in the perovskite phase [23]. In oxynitride glasses, La3+ does not incorporate into any crystals, but the Vickers hardness is increased with La doping, because of the high cationic field strength of La3+ and its effect in the residual glass [24–26]. In Co2+-doped MgO-Al2O3-SiO2 glass ceramics,

Corresponding author. E-mail address: [email protected] (B. Li).

https://doi.org/10.1016/j.jnoncrysol.2019.119638 Received 7 May 2019; Received in revised form 13 August 2019; Accepted 20 August 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 524 (2019) 119638

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2.2. Characterization

La2O3 does not participate in the crystal phase [27]. In summary, La2O3 shows a variety of distinctive effects in different glass ceramic systems depending on its specific behavior in that system. For CaO–MgO–Al2O3–SiO2 (CMAS) glass ceramics, augite and diopside (both clinopyroxenes) are normally the main crystal phases. Diopside (CaMgSi2O6) and hedenbergite (CaFe2+Si2O6) are endmember clinopyoxene, while augite can be described as (Ca,Na) (Mg,Fe,Al,Ti)(Si,Al)2O6. Kansal [28] reports that augite is the main phase in CMAS glass ceramics, but does not discuss whether La3+ is dissolved in the augite phase. Goel [29] presumes that La3+ enter the augite phase when Cr2O3 is added. Detailed studies are still needed to understand the effect of La2O3 in CMAS glass ceramics. Therefore, this work aims to assess the effect of La3+ on the microstructure, micromorphology, crystalline phases, and residual glass, and its possible interaction with other network modifiers in CMAS glass ceramics.

2. Materials and

DSC measurements were performed on a NETZSCH STA 449C in air, at the heating rates of 10 °C/min with reference to Al2O3, to assess the glass-transition and crystallization temperatures of the parent glass. The crystalline phases and relative concentration in the glass ceramics after heat treatment were determined by a PANalytical X'pert PRO Powder XRD. Cu Kα (1.541874 Å) radiation at 40 mA and 40 kV was used as the source. Quantitative phase analysis was performed by Rietveld refinement using Highscore software, using a 20 mass% corundum Al2O3 internal standard. Scanning electron microscopy (SEM) - backscattered electron (BSE) images and qualitative energy dispersive X-ray spectra (EDS) of TLax glass ceramic were obtained using a Carl ZEISS SUPRA 55 Field Emission Scanning Electron Microscope (FESEM). High-angle annular dark-field (HAADF) images, high-resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED), and EDS spectra of TLa0, TLa 2, and TLa5 samples were determined using a FEI Tecnai G2 F20 TEM at 200 kV. Sample density was measured using the Archimedes principle. The heat-treated glass ceramic samples were cut into 3 mm × 4 mm × 40 mm strips with a diamond grinding wheel cutting machine, and the bending strength of the samples was measured on a CSS-88000 electronic universal testing machine by a three-point bending method. The hardness of the polished samples was measured on an Hv-50A Vickers hardness tester by the indentation method. The acid/alkali-resistance corrosion test was evaluated with the standard JC/T258-1993 [31]. The heat-treated glass-ceramic samples were crushed, and particle sizes between 0.0197 and 0.0394 in. were used for the test. One gram of crushed glass-ceramic particles was placed in an Erlenmeyer flask with H2SO4 (20 wt%) or NaOH (20 wt%), and heated in a 100 °C water bath for one hour. The glass particles were removed, washed and dried, and the weight loss of the particles after corrosion was measured. The weight loss rate was determined by measuring the weight loss at different times. To evaluate the density, Vickers hardness, bending strength and resistances to both acid and alkali corrosion, six samples were used in each measurement, and the averaged results were reported with their standard deviations in this study.

methods

2.1. Raw materials and preparation The Bayan Obo west mine tailing and locally collected fly ash are used as the main raw materials. Their chemical compositions are listed in Table 1. There are still 1.54 wt% rare earth oxides (REO) left in the tailing due to current extraction process limitations. Table 2 presents the contents of the REO, in which the weight percent (wt%) of four REO, namely La2O3, CeO2, Pr2O3, and Nd2O3, normally accounts for nearly 96% of the total. Based on a previous study [30], an optimized tailing glass-ceramic composition of 50.00 SiO2–20.00 CaO-5.57 Al2O3–7.13 MgO (wt%) was chosen as the baseline parent glass. Cr2O3 and CaF2 were chosen as the nucleating agents. Na2O and K2O were added to reduce the glass viscosity. The optimized parent glass is derived from the efficient use of Bayan Obo West Mine tailing and fly ash. The total utilization (waste loading) of Bayan Obo West Mine tailing and fly ash is ≈60 wt%. SiO2, CaO, and MgO chemicals were also added to adjust the parent glass composition. Six compositions, denoted TLa0, TLa1, TLa2, TLa3, TLa4 and TLa5, have been produced with additions of La2O3 of 0, 1.0, 2.0, 4.0, 6.0 and 8.0 wt%, respectively. For each TLax (x = 0–5), glass powder was mixed in a ball mill for 1 h to prepare a 200 g batch for melting. The six batches were simultaneously melted in separate corundum (Al2O3) crucibles at 1450 °C for 2.5 h. The clarified frits were cast into 6 cm × 4 cm × 8 cm stainless steel molds which were preheated to 600 °C. The solidified glass plates were removed from the molds transferred into a muffle furnace to anneal at 600 °C for 4 h, after which the furnace was turned off and, the samples cooled to room temperature in the furnace. A fraction of the above-mentioned homogeneous melts were quenched into water for subsequent differential scanning calorimetry (DSC) measurements. According to the nucleation and crystallization temperatures determined by DSC results, the annealed samples were heated in air at 3 °C/min to nucleation temperature and 2 °C/min to crystallization temperature, dwelled for 2 h each at the nucleation and crystallization (growth) temperatures, then cooled to room temperature inside the furnace with no specific thermal control.

3. Results and discussion 3.1. Glass ceramics formation Fig. 1 shows the DSC curves for six TLax (x = 0–5) water-quenched samples. Compared with TLa0, the glass transition temperature (Tg) of TLa1 sample increased from 665 °C to 675 °C. Further increase of La2O3 content from 1.0–8.0 wt%, tended to stabilize the Tg around 675 °C for all La2O3-doped samples TLa1 to TLa5. The crystallization temperature (TP) corresponding to the exothermic peak shifted to higher temperature with La2O3 doping. TP systematically increased from 865 °C (TLa0) to 902 °C (TLa5), except the TLa3 sample (882 °C). By contrast to the TLa0 (La2O3-free) sample, both Tg and TP of all La2O3-doped samples shifted to higher temperature, which is attributed to the higher bond energy of LaeO (244 kJ/mol) than that of CaeO (134 kJ/mol) [32]. The identical Tg for five La2O3-doped samples is due to the high

Table 1 Chemical compositions of Bayan Obo west mine tailing and fly ash (wt%). The stated values are typically ± 0.10 to ± 0.50 as determined by four repeated measurements. Raw materials

SiO2

CaO

Al2O3

MgO

Na2O

K2O

MnO

Fe2O3

FeO

Bayan Obo West Tailing Fly ash

20.70 51.68

26.75 3.89

1.93 30.70

14.03 0.76

1.23 1.84

1.61 1.62

3.28 0.12

13.60 7.43

5.80 –

*REO: rare earth oxide. –: none or negligible. *LOI: loss on ignition at 1000 °C. 2

CaF2

*REO

*LOI

5.00

1.54

–

–

4.53 1.96

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Table 2 Contents of rare earth oxides in Bayan Obo west mine tailing (wt% of total REO). The stated values are typically ± 0.10 to ± 0.50 as determined by four repeated measurements. REO

La2O3

CeO2

Pr2O3

Nd2O3

Sm2O3

Eu2O3

Gd2O3

Tb2O3

wt%

24.44

48.46

6.59

14.57

1.82

0.29

1.46

0.17

REO

Dy2O3

Ho2O3

Er2O3

Tm2O3

Yb2O3

Lu2O3

Y2O3

wt%

0.25

< 0.10

0.20

< 0.10

< 0.10

< 0.10

1.63

Fig. 1. DSC curves of the parent glasses with 0–8 wt% La2O3.

strength LaeO bond which is difficult to break at the glass transition temperature. Similarly, around the crystallization temperature, LaeO bond breakage requires more energy than CaeO bonds. The addition of La2O3 can reduce the non-bridging oxygen atoms in a silicate glass, so the glass structure was also stabilized by La2O3-doping. This result is consistent with other literature [29,32]. The interval between TP and Tg (ΔT = TP − Tg) can also be used as an indicator of the thermal stability of the glass ceramics, i.e., the bigger the interval, the more stable the glass ceramic to unwanted devitrification, allowing precise and independent control of the nucleation and growth regimes [26,33]. As shown in Fig. 1, ΔT increases from 200 °C to 225 °C with increasing La2O3 content from 0 to 8.0 wt%, which indicates the La2O3-doped glasses have a better stability against spontaneous crystallization, thus better control is available for heat treatment [32]. This provides a good basis for commercial applications of glass ceramics. The optimum nucleation for these glass ceramics occurs at 50–100 °C higher than Tg, and the crystallization process mainly occurs near the exothermic peak temperature [34]. According to the DSC results, the nucleation and crystallization temperatures of TLa0-TLa5 samples were determined to be 720 °C and 880 °C, respectively. The TLa0 to TLa5 glass-ceramic samples, after heat treatment at 880 °C for two hours, are cut into the required size, and the internal sections are shown in Fig. 2. All six samples are dark green and opaque, as all samples crystallized in a bulk manner instead of the one starting from the surface. There are not any pores shown in the cut sections. Only the TLa1 sample has some phase segregation due to uneven composition. Fig. 3 shows the XRD patterns of heat-treated TLax (x = 0–5) glassceramic samples. Augite [(Ca0.742Fe0.087)(Mg0.016Al0.888Fe0.075) (Al0.5Si1.5)O6, monoclinic, space group C2/c] is detected as the primary crystalline phase in all six samples. A new phase, lanthanum oxyapatite [CaLa4(SiO4)3O, hexagonal, space group P 63/m], is detected in the samples with La2O3 doping. There is still a small amount of fluorite (CaF2) which either did not dissolve or reprecipitated. When the La2O3 content is higher than 4.0 wt% (i.e., TLa3-TLa5 samples), the diffraction peak intensity of the lanthanum oxyapatite becomes stronger, while that of augite becomes weaker. In order to clarify the effect of the La3+ cation on the crystallinity, a

Fig. 2. Cross-section images of the glass ceramics (TLa0 -TLa5) after heat treatment at 880 °C for two hours.

Fig. 3. XRD patterns of all six glass ceramics TLax (x = 0–5) after heat treatment. Table 3 Crystal and amorphous fractions (wt%) in the TLa0 to TLa5 glass ceramics. Using the methods established previously [35], the error on the phase fraction was estimated to be ± 0.07× the wt% value for augite and amorphous phases and ~8× the stated value for the lanthanum oxyapatite phase < 2 wt%, and ~2× for the lanthanum oxyapatite phase 2–5 wt%. Sample

TLa0 TLa1 TLa2 TLa3 TLa4 TLa5

3

Crystallinity Augite

Lanthanum Oxyapatite

Amorphous

54.7% 60.2% 76.8% 54.5% 50.9% 50.8%

–

45.3% 39.7% 23.0% 45.3% 47.7% 45.1%

0.1% 0.2% 0.2% 1.4% 4.1%

Journal of Non-Crystalline Solids 524 (2019) 119638

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Table 5 Ionic radii of selected elements in glass ceramics [42]. Elements

Ca2+

La3+

Si4+

Mg2+

Al3+

Fe3+

Na+

Ionic radii for 8 CN (Å) Ionic radii for 6 CN (Å)

1.120 1.000

1.160 1.032

–

0.890 0.720

–

0.780 0.645

1.180 1.020

0.400

0.535

substitution is very common in augite phases. Two conditions are required to form a complete substitution solid solution. The first condition is that the substitute ions must be similar in ionic size. The second is that there must be charge neutrality; in the simplest case, ions must have the same charge, such as Fe2+ substituting for Mg2+ on the M1 site. If the substitute ions have different ionic charges, a coupled substitution is required to balance the charge; an example is Na+ substituting for Ca2+ in the M2 site, coupled with a Al3+ substituting for Si4+ [39]. La3+ is a large ionic radius ion, so it can only form a limited solid solution. The ionic radius of La3+ is compared with the other ions typically present in augite in Table 5, showing that La3+ has a similar ionic radius to Ca2+. Although La3+ has a different charge than Ca2+, during a coupled substitution, such as Al3+ for Si4+ or Na+ for Ca2+, overall charge balance is maintained [40]. Such substitution is very common in CaO-MgO-Al2O3-SiO2 [37] and Na2O-P2O5-Bi2O3-TiO2 glass systems [41]. As previously stated, the LaeO bond energy (243 kJ/mol) is much higher than the bond energy of CaeO (134 kJ/mol). Therefore, the La3+ ion substitution into the augite phase or into the residual glass will improve the mechanical properties of the glass ceramic. Because La3+ and Ca2+ are heterovalent substitutions, La3+ cannot replace Ca2+ indefinitely. Thus, when the amount of La3+ doping exceeds the solid solubility in the augite phase, a second phase is precipitated. Therefore, it is clear that with the increasing amount of La2O3, La3+ first replaces Ca2+ in augite to form a substitutional solid solution. This result confirms the hypothesis that La2O3 enters into pyroxene with a small amount of Cr2O3 [29].

Fig. 4. Partial XRD patterns of TLax (x = 0–5) glass ceramics within the 2θ range of 28–33°.

quantitative phase analysis of heat treated TLa0 to TLa5 samples was performed by Rietveld refinement; the results are shown in Table 3. Compared with TLa0, the crystal fraction of augite increases to 60.2% (TLa1) and 76.8%(TLa2). When La2O3 content further increases from 4 to 8 wt%, the crystallinity of augite rather decreases from 54.5% (TLa3) to 50.8% (TLa5), and the fraction of lanthanum oxyapatite increases from 0.2 to 4.1%. This result implies that a small amount of La2O3 promotes the precipitation of the augite phase. However, when the La2O3 content is > 4 wt%, the precipitation of the lanthanum oxyapatite phase somewhat suppresses the formation of augite. Therefore, the precise amount of La2O3-doping has a significant effect on the fraction of crystal phases, even though La2O3-doping does not change the identity of the primary crystalline phase (i.e., augite in this case). This result is consistent with the role of La2O3 in ferroelectric barium strontium titanate glass ceramics as reported by Chen [21]. In order to reveal the effect of the La3+ cation on the augite structure, XRD patterns between 28° and 33° (2θ) are presented in Fig. 4, and the cell parameters for augite are obtained by Rietveld refinement (Table 4). The value of Rietveld reliability factors (Rwp, Rp, and Rexp) are all < 7; these low values indicate a good fit to the experimental results and highly reliable phase quantification. With increasing La2O3, the main peaks {221} of augite are consistently shifted to lower 2θ angles (Fig. 4), and the a, b, and c lattice parameters and the unit cell volume of augite become consistently larger (Table 4). All six samples (TLa0-TLa5) are fabricated under the same experimental conditions. Therefore, the observed diffraction peak of augite which shifts with La2O3-doping, can only be caused by a fraction of La3+ ions entering into the augite crystal structure. Augite belongs to the family of single-chain silicates, as its [SiO4] tetrahedra form an infinitely extended single chain along the c-axis in an apical corner-sharing manner. [Si2O6] single chains are linked by metal cations [36]. The augite structural formula is M2M1Z2O6, where M2 contains Ca2+, Mg2+, Fe2+, Mn2+ or Na+ in the distorted 8-coordinated site. M1 contains Mg2+, Al3+, Fe2+, Fe3+, Cr3+, or Ti4+ in the regular octahedral (6-coordinated) site, and Z contains Si4+ and Al3+ in the tetrahedral (4-coordinated) sites [37,38]. The augite phase in glass ceramics can manifest in many solid solutions, since ion

3.2. Microstructure Fig. 5 shows the SEM-BSE images of six heat-treated TLax (x = 0–5) samples, along with the EDS elemental mapping of TLa5. In the BSE images, dark backgrounds are glass matrices. Based on the EDS elemental maps of TLa5, dendrite-type structures are enriched in Si, Mg, Al, Fe, Ca, O elements. Spherical-shaped structures are enriched in Ca and F. Needle-type structures are enriched in La, Ca and O. According to XRD results, the gray dendritic crystals are augite. Gray sphericalshapes are fluorite (CaF2). With La2O3 doping, white needle-type lanthanum oxyapatite with Cr-doping are observed, with their content increasing dramatically in TLa4 and TLa5 samples. Here the lanthanum oxyapatite crystals appear clustered in the gaps between the dendrites of the augite primary crystal phase. In order to further confirm the influence of lanthanum ions on the microstructure of the crystalline phases and residual glass, TLa0, TLa2, and TLa5 samples were examined in TEM. Fig. 6 presents STEMHADDF micrographs of the La2O3-free sample (TLa0) after heat

Table 4 Rietveld refined structures based on the XRD data of sample TLa0 - TLa5. Sample

Unit cell parameters of Augite a (Å)

TLa0 TLa1 TLa2 TLa3 TLa4 TLa5

9.7426 9.7445 9.7448 9.7498 9.7521 9.7564

Volume

b (Å) ± ± ± ± ± ±

0.0038 0.0039 0.0042 0.0049 0.0056 0.0059

8.8680 8.8716 8.8718 8.8787 8.8846 8.8901

c (Å) ± ± ± ± ± ±

0.0032 0.0032 0.0035 0.0041 0.0047 0.0048

5.3082 5.3082 5.3094 5.3138 5.3153 5.3196

± ± ± ± ± ±

0.0021 0.0022 0.0022 0.0027 0.0029 0.0030

4

α = γ(°)

β (°)

90.000 90.000 90.000 90.000 90.000 90.000

106.1221 106.0946 106.0852 106.0673 106.0181 106.0114

Å3 ± ± ± ± ± ±

0.0006 0.0007 0.0007 0.0008 0.0009 0.0011

440.5789 440.9082 441.0491 442.0247 442.6521 443.5036

± ± ± ± ± ±

0.0051 0.0056 0.0057 0.0062 0.0071 0.0072

Journal of Non-Crystalline Solids 524 (2019) 119638

H. Chen, et al.

Fig. 5. BSE-SEM image of TLa0 to TLa5 glass ceramics; corresponding EDS elemental maps of TLa5.

8.0 wt% La2O3, after heat treatment at 880 °C for 2 h. According to the XRD patterns (Fig. 3) and EDS point spectrum (Fig. 8d–f), there are three crystal phases in TLa5, which are augite, lanthanum oxyapatite, and fluorite. Compared with TLa0 and TLa2 samples (Figs. 6–7), the size of the augite phase has significantly increased. Based on the EDS spectrum of augite (Fig. 8d), lanthanum does substitute into the augite phase, as suggested by XRD patterns. The Rare Earth-rich phase lanthanum oxyapatite still exhibits a skeletal-type or spherical growth, and the crystal fraction of lanthanum oxyapatite is obviously increased. Fluorite presents as a block growth morphology. There is no apparent crystallographic growth dependence among the three crystal phases. Fig. 8b presents the HRTEM of the residual glass of TLa5 sample. Unlike the residual glass phase of La2O3-free TLa0 sample (Fig. 6c), fine lattice fringes appear in the HRTEM of the residual glass phase of sample TLa5. This suggests that nano crystal clusters appear in the residual glass phase when lanthanum is added. Based on the EDS point scan of the crystal clusters and the rest of the residual glass phase (Fig. 8c, g), the composition of the crystal cluster is very similar to that of the augite phase. The residual glass phase is mainly composed of Si, Al and O, with Ca, Na, Fe, and Mg being in low concentration. Therefore, the 2–5 nm crystal clusters can be considered to be nanosized augite crystals. Because the high cationic field strength of La3+, the other metal ions Fe3+, Ca2+, and Mg2+ are prone to cluster around La3+ ions in the glass to form augite crystals. Since these metal ions

treatment at 880 °C for 2 h, and the corresponding HRTEM image and SAED pattern. Base on lattice fringes in the HRTEM image, the SAED pattern (Fig. 6b) and the EDS point spectrum (Fig. 6d), it is confirmed that the augite phase in Fig. 6a grows in a dendritic form in the lanthanum-free glass ceramic prepared from Bayan Obo West Mine tailing. Fig. 6c shows the HRTEM image of the residual glass. Atoms in the residual glass are randomly arranged, and there are no obvious lattice fringes appearing in the HRTEM image. Fig. 6e verifies the residual glass is enriched in Si, O and Al, and also contains some Fe, Mg, Ca, and Na. Fig. 7 presents the TEM micrograph of the TLa2 sample, containing 2.0 wt% La2O3, after heat treatment at 880 °C for 2 h. There are two crystal phases in Fig. 7a. One is augite, which exhibits dendritic growth; another is a white skeletal-type or grain-type phase (Fig. 7a). In order to further confirm the crystal structure of the white skeletal-type second phase, HRTEM and SAED are taken from two different orientations. Based on lattice fringes in HRTEM images (Fig. 7.b, d), FFT (Fast Fourier transform) patterns (Fig. 7b, e), SAED patterns (Fig. 7c, f), and XRD results (Fig. 3), the white skeletal-type phases are lanthanum oxyapatite. Lanthanum oxyapatite crystals are mainly present in the residual glass. Some lanthanum oxyapatite crystals are in contact with the augite, but there is no obvious growth dependence or relationship between the lanthanum oxyapatite and augite phases. Fig. 8 presents a TEM micrograph of the TLa5 sample, containing 5

Journal of Non-Crystalline Solids 524 (2019) 119638

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Fig. 6. TEM micrograph of TLa0 sample along with EDS point spectra. (a) STEM-HADDF image; (b) HRTEM image and SAED patterns of augite; (c) HRTEM image of residual glass; (d) EDS spectrum of augite phase; (e) EDS spectrum of residual glass.

Fig. 7. TEM micrograph of TLa2 sample. (a)STEM-HAADF image; (b) HRTEM image and FFT pattern of lanthanum oxyapatite; (c) SAED pattern of lanthanum oxyapatite; (d) HRTEM image of lanthanum oxyapatite with different orientation; (e) FFT pattern extracted from HRTEM image of (d); (f) SAED pattern of lanthanum oxyapatite.

6

Journal of Non-Crystalline Solids 524 (2019) 119638

H. Chen, et al.

Fig. 8. TEM micrograph of TLa5 sample and corresponding EDS spectra. (a) STEM-HAADF image; (b) HRTEM image of residual glass phase; (c) EDS spectra of imaged point crystal cluster and residual glass from Fig. 8(b); (d) EDS spectrum of augite; (e) EDS spectrum of lanthanum oxyapatite; (f) EDS spectrum of fluorite; (g) EDS spectrum of residual glass.

7

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compact than when La2O3-free, which also makes the glass density increase. The heat treated glass-ceramic density is greater than the annealed glass density for each pair, which reflects the fact that the glass-ceramic molar volume must be smaller than the corresponding glass, since the overall composition is the same. The mechanical characteristics of the glass-ceramic samples were evaluated by Vickers hardness and bending strength. Fig. 10 shows the Vickers hardness and bending strength of the heat-treated TLax (x = 0–5) glass-ceramic samples. The compositions of this series of samples are designed on the basis of a previously determined optimal composition, so all Vickers hardness and bending strengths of the six samples are relatively high. Vickers hardness varied between 7.22 and 7.82 GPa, higher values than obtained with previous pyroxene glass ceramics 6.08 GPa [37]. Bending strength varied between 206 and 238 MPa, except the TLa5 which was lower. These values are higher than those of the glass ceramics made from blast furnace slag which averaged 115 MPa [5], or the one fabricated from metallurgical ferronickel wastes which averaged 120 MPa [44]. Glass ceramic is generally a brittle material. Its Vickers hardness depends on the composition, amount, distribution, and morphology of the crystal phases [45]. Compared with La2O3-free samples, Vickers hardness increased in all La2O3-doped samples (TLa1 to TLa5). The morphology changed slightly in TLa1 and TLa2 samples compared to the TLa0 sample. The addition of La2O3 increased the crystallinity of augite phase in TLa1 and TLa2 samples, so this is the reason that the Vickers hardness increased in these two samples. Vickers hardness not only depends on the microstructure of the crystal phases, but also depends on the interaction between crystal and residual glass, because the boundaries between glass and crystal and between crystals are the weakest regions. The smaller the crystal grain size is, the longer the path required for crack propagation. The deflection, bending, and transfer of cracks can consume the energy of crack propagation. Therefore, smaller crystals help to prevent crack propagation. Augite is the primary crystal phase in all six samples, and it exhibits dendritic growth in all cases. The crystal size of augite in TLa3 is smaller than all others. That is the reason the TLa3 sample has the highest Vickers hardness. As La2O3 is further increased, although the crystal fraction of augite phase is decreased, the augite formed coarse dendrites in TLa4 and TLa5 samples. The addition of La2O3 increased the fraction of lanthanum oxyapatite phase, which is good for restricting the expansion of cracks. Therefore, the Vickers hardness of TLa4 and TLa5 samples still high. The TLa2 sample has the highest bending strength, which is obviously related to the highest crystallinity and the interlocked growth of

Fig. 9. Density of annealed glass and heat-treated TLa0 to TLa5 glass ceramics. The averaged results and the standard deviations were determined by one sample for six repeated measurements.

have decreased concentration in the residual glass, the glass network polymerization may be increased. This result can become an important factor contributing to the enhancement in the strength and hardness of the residual glass and its resistance to acid/alkaline attack. 3.3. Physical and chemical properties Fig. 9 shows the density of TLax (x = 0–5) annealed glasses and heat-treated glass ceramics. The density of the annealed glasses varied between 2.85 g/cm3 and 3.03 g/cm3, and the density of heat-treated glass ceramics varied between 3.02 g/cm3 and 3.17 g/cm3. The density of both annealed glass and heat-treated glass ceramics increased with increasing the concentration of La2O3, which agrees with literature [19,27]. The density of the glass depends on the molecular weight and its content of the constituent glass oxides, as well as the compactness of the glass network [43]. The addition of the La2O3 makes the density of both annealed and heat-treated glass ceramics gradually increase, which is primarily related to the molecular weight of the La2O3 being larger than the other the main component in the glass. At the same time, La2O3 is also a glass network modifier. Due to the strong electric field and high coordination number of La3+, the glass network containing it is more

Fig. 10. Vickers hardness and bending strength of TLa0 to TLa5 glass ceramics. The averaged results and the standard deviations were determined by six samples for each measurement.

Fig. 11. Histogram on acid/alkali-resistance of TLa0~TLa5 glass ceramics. The averaged results and the standard deviations were determined by six samples for each measurement. 8

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Table 6 Physical and chemical properties of glass–ceramics made from wastes. Raw material

Density (g/cm3)

Bending strength (MPa)

Bayan Obo mine tailing (TLa2) Blast furnace slag [5] Fe-Ni metallurgical wastes [44] Coal fly ash and waste glass [47] Iron-making melter slag [48] Waste glass and fly ash [2]

3.06 ± 0.02 3.02 3.16 ± 0.01 2.49 3.1 1.94

238 ± 19 115 120 ± 9 94.1 184 ± 26 78

a

Vickers hardness (GPa)

Acid-resistance 20%H2SO4

Alkali- resistance 20%NaOH

7.57 ± 0.21

97.52 ± 1.30% 91.5% TCLP testa 86.6%

98.93 ± 1.41% 91.5% TCLP test

–

–

–

–

–

9 ± 0.2 5.30 ± 0.04 7.8 ± 0.2 –

–

TCLP: Toxicity Characteristic Leaching Procedure.

These microstructural changes in turn lead to the physical and chemical property variations as present in Figs. 10 and 11.

the augite phase. The acid/alkali-resistance histogram of TLa0 to TLa5 glass ceramic is shown in Fig. 11. With the La2O3 doping, the acid-resistance increased and reached the maximum of 97.52% in the TLa2 sample. With La2O3 further increased, the acid-resistance of glass-ceramics decreased from 95.88% (TLa3) to 91.34% (TLa4) and then increased to 93.62% (TLa5). The alkali-resistance of six glass-ceramics is very high, around 98%. The corrosion mechanism of glass ceramics by H2SO4 is the ion exchange reaction between H+ cation and metal cations in the surface, especially with metal ions in the glass phase, such as Fe3+, Ca2+, Mg2+, Al3+ and Na+ cations [4]. The change in acid-resistance of TLa0 to TLa5 samples is very similar to its overall change in crystallinity; the higher the crystallinity, the more metal cations are incorporated in crystalline phases. Therefore, fewer cations are available to ion exchange with the H+ ion, and the acid resistance is higher. Furthermore, H2SO4 could form micro cracks on the surface by ion exchange reaction, and then corrode the inside of the sample along the cracks [46]. A microstructure with large dendritic crystals is more prone to the expansion of micro cracks in the glass phase due to thermal expansion mismatch. This is the reason that acid-resistance of TLa4 and TLa5 samples is lower. The alkali-resistance does not show significant composition dependence, and is high (97–99%) for all glass-ceramic samples. The physical and chemical stability properties of some glass-ceramics made from different raw materials are given in Table 6. It shows that the La2O3 (TLa2) sample in this study has shown general superiorities over the glass ceramics made from blast furnace slag, FeeNi metallurgical wastes, iron-making slag, waste glass, and fly ash, in terms of the bending strength, hardness, and chemical durability. The results presented above have shown that progressive increase of La2O3 can effectively change both the composition and morphology of the crystalline phases and the residual glass of the CMAS glass ceramics. The Rietveld analysis on the XRD patterns shown in Fig. 4, further quantitatively confirmed the partial substitution of La3+ for Ca2+ in the main crystalline phase, i.e. augite. La2O3 is thus shown to improve the crystallization of augite in additions up to 2 wt%, and these microstructural changes induced variations in the Vickers hardness, bending strength, and chemical resistance of the CMAS glass ceramics. Results from XRD (Fig. 4), SEM and the corresponding EDS (Fig. 5), and TEM (Fig. 8), have shown that high concentration doping La2O3 of > 2 wt% leads to an increasing amount of needle-like lanthanum oxyapatite phases within the residual glass formed between augite grains. Similar phenomena have also been reported in the MgO-Al2O3-SiO2-TiO2 glassceramics containing > 0.3 mol% La2O3 [49]. The EDS result in Fig. 5. shows this new secondary phase contains some Cr. Thus the increasing formation of such La-Cr containing secondary phase should lead to a decrease in the formation of Cr-related spinel phase. As it has been introduced earlier, such spinel phase has been wide reported to be one of the most effective nuclei to grow augite crystals. This microstructural change may reduce the amount of Crspinel nuclei and that of augite crystals latterly formed on the Cr-spinel nuclei. Therefore augite crystals in smaller volumetric concentration can grow into dendrites with a larger average size as presented in Fig. 5.

4. Conclusions A batch of calcium-magnesium-aluminum-silicate (CMAS) glass ceramics with 0, 1.0, 2.0, 4.0, 6.0 and 8.0 wt% La2O3 were fabricated and crystallized by nucleation and crystal growth processing. The structures of the crystal phase and residual glass were changed by La2O3-doping. The crystallinity of augite was increased by adding La2O3 up to 2 wt%. Lanthanum oxyapatite was formed with higher La2O3-doping. Through XRD and TEM, it was confirmed that lanthanum ions can substitute for calcium in augite. Sodium, magnesium, aluminum, calcium and iron ions in the residual glass phase formed nanometer-sized crystal clusters of augite composition, due to the influence of La3+. The bending strength and Vickers hardness of the glass ceramics reached 238 MPa and 782 GPa, respectively. The maximum acidresistance and alkali-resistance were 97.52% and 98.93%. The engineering properties of these glass ceramics, synthesized using Bayan Obo West Mine tailing and fly ash, were thereby suitable for to be used as low-price anti-erosion and anti-abrasion materials. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the residue fund of the National Program on Key Basic Research Project 973 Program (Grant No. 2012CB722802), the National Natural Science Foundation of China (Grant No. 11564031, 51774189), and the “Inner Mongolia Autonomous Region Science and Technology Major Project: Fundamental and key technology research for the integrated exploitation of Bayan Obo Mine with high added value”. This study was also funded by the Natural Science Foundation of Inner Mongolia Autonomous Region (Grant No. 2018MS05033). The authors thank Chao Ma assistance with Scanning Transmission Electron Microscope and José Marcial for help with the Field Emission Scanning Electron Microscope. References [1] D. Huang, C.H. Drummond, J. Wang, R.D. Blume, Incorporation of chromium(III) and chromium(VI) oxides in a simulated basaltic, industrial waste glass-ceramic, J. Am. Ceram. Soc. 87 (11) (2004) 2047–2052. [2] Z. Lu, J. Lu, X. Li, G. Shao, Effect of MgO addition on sinterability, crystallization kinetics, and flexural strength of glass-ceramics from waste materials, Ceram. Int. 42 (2) (2016) 3452–3459. [3] W.L. Chen, X.F. Zhang, X.L. Jia, W.C. Xu, Y.L. Liu, B.W. Li, Pressure-induced preferential grain growth, texture development, and anisotropic properties of Fe/augite matrix composites prepared by spark plasma sintering, Mater. Res. Express 5 (9) (2018) 95202. [4] H. Zhang, Y. Du, X. Yang, X. Zhang, M. Zhao, H. Chen, S. Ouyang, B. Li, Influence of

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